The present invention relates generally to the production of curved, tempered sheets of glass and, more particularly, to an improved method of and apparatus for bending and heat treating relatively thin glass sheets.
Bent glass is widely used in glazing side openings in vehicles such as automobiles or the like and, to be suitable for such application, flat glass sheets or plates must be bent to precisely defined curvatures dictated by the overall styling of the vehicles and the manner of mounting the glass in the openings. At the same time it is important that the bent sheets meet stringent optical requirements and that the viewing area of the window or light be free of optical defects that would tend to interfere with the clear viewing therethrough. Further, bent glass sheets intended for use as glazing closures in vehicles are subjected to an additional heat treatment to temper the glass for strengthening the same and increasing the resistance thereof to damage resulting from impact.
In general, therefore, the commercial production of bent glass sheets for such purposes commonly includes heating flat sheets to the softening point of the glass, bending the heated sheets to a desired curvature and thereafter cooling the bent sheets in a controlled manner to a temperature below the annealing range of the glass.
For reasons of efficiency and to permit their being handled in large quantities, the sheets are conventionally heated, bent and cooled while being moved almost continuously along a fixed path and successively through a heating area, a bending area and a cooling area. To achieve satisfactory temper, the temperature of the glass sheet must be above a predetermined minimum level so as to maintain the core or interior thereof above a deformation temperature upon being exposed to the tempering medium. The residual heat remaining in glass sheets of conventional thicknesses, such as those having thicknesses ranging from 0.180 inch to 0.255 inch for example, is generally above such predetermined minimal level after bending for immediate advancement to the tempering area and exposure to the tempering medium. Thus, the heat initially imparted to the sheet to bring it to proper bending temperature can also be utilized in the final heat treating operation.
However, within the last several years, considerable emphasis has been placed on the use of thinner and thinner glass sheets for automotive glazing purposes. This has posed problems in bending and tempering, due to the lesser ability of the thinner sheets to retain heat and the aforementioned conventional process of bending and treating glass sheets does not lend itself to the processing of these relatively thin sheets, such as those having thicknesses ranging from 0.090 to 0.156 inch for example. As the thickness of the glass decreases, the rate of heat loss increases and the heat initially imparted to such thin sheets is quickly dissipated upon leaving the heating atmosphere of the furnace and during the relatively cool bending cycle. Attempts to solve these problems by initially overheating the thin sheets have not been successful because of the consequent loss of deformation control and the degradation of the surface quality of the finished glass as a result of heat stains, rolls distortion, pitting and the like.